The present invention relates to the field of metallurgy, and more particularly to corrosion-resistant and wear-resistant steel.
Various kinds of steel are known in the art, that are used in the manufacture of products related to medical engineering, e.g., prostheses, implants, medical tools, and the like. Steels used for the indicated purpose must meet definite requirements both from the standpoint of the interaction of products made from such steels with human organism and from the standpoint of the physico-mechanical characteristics thereof.
However, the strength and wear-resistance of the known steels used for medical purposes are not sufficient, and therefore prostheses, implants, and medical tools manufactured from the known steels cannot insure the required safety and reliability in the course of their operation and rapidly wear out. In the case of prolonged contact with human tissues the known steels used for medical purposes induce allergic reactions of human organism.
To the category of steels used for medical purposes there belong corrosion-resistant austenitic steels described in the International Translator of Modern Steels and Alloys (Prof. V. S. Kershenbaum (Ed.), xe2x80x9cInternational Engineering Encyclopediaxe2x80x9d Series, Moscow, 1992 (in Russian)), for instance, steel grade AISI 316H (USA), containing 0.04-0.10% by weight of carbon, 16.0-18.0% by weight of chromium, 10.0-14.0% by weight of nickel, 2.03.0% by weight of molybdenum, 1.0% by weight or less of silicon, 2.0% by weight or less of manganese, 0.03% by weight or less of sulfur, the balance being iron, or steel grade DIN 17440 (DE), containing not more than 0. 1% by weight of carbon, not more than 1.0% by weight of silicon, not more than 2.0% by weight of manganese, not more than 0.045% by weight of phosphorus, not more than 0.03% by weight of sulfur, 16.5-18.5% by weight of chromium, 12.0-14.0% by weight of nickel, less than 5.0% by weight of titanium and 2.0-3.0% by weight of molybdenum.
Said steels have a single-phase austenitic structure, but they contain an appreciable amount of critical and costly nickel which, among other things, may induce allergic reactions in human organism when said steels are used for medical purposes; furthermore, said steels contain manganese which reacts with human blood.
Besides, said steels have a low strength ("sgr"B being less than 520 MPa, "sgr"0.2 being less than 250 MPa) and an insufficient wear-resistance, so that they fail to meet the requirements to the materials for products to be used in medical engineering.
Closest in its chemical composition to the steel of the present invention is the corrosion-resistant steel described in EP No. 0123054, 06.05.1987. This steel contains, in % by weight: carbon, from 0.01 to 0.5; chromium, from 3.0 to 45.0; niobium, up to 10.0; silicon, up to 2.0; manganese, up to 0.10; molybdenum, up to 10.0; vanadium, up to 5.0; titanium, niobium and/or tantalum, up to 2.0; cerium, up to 1.0; aluminum, up to 0.3; nitrogen, from 0.2 to 5.0; iron, the balance.
Said steel has an austenitic-ferritic structure and is magnetizable. At 400xc2x0 C. said steel has the yield point Rp 0.2 ("sgr"0.2) less than 400 N/mm2 and at 600xc2x0 C. it has the yield point Rp 0.2 ("sgr"0.2) less than 250 N/mm2. The described steel is intended, owing to its heat resistance, for manufacturing gas and steam turbines.
The above-indicated steel is not suitable for manufacturing products to be used in medical engineering, because, in the first place, its structure comprises at least 50% of ferromagnetic components able to react with human blood containing iron ions; in the second place, said steel contains manganese and nickel which induce allergic reactions when in contact with human tissues.
The present invention is directed to the provision of a nonmagnetic steel which has a high mechanical strength, high corrosion and wear resistance, plasticity and is inactive with respect to human tissues.
Said object is accomplished in the provision of a steel containing carbon, chromium, silicon, manganese, nitrogen and iron, which steel, according to the invention, contains said 10 components in the following relationship, % by weight:
the total content of ferrite-forming components in the steel, namely, of silicon and chromium, and the total content of aus-tenite-forming components therein, namely, of carbon, nitrogen 20 and manganese, obeying the following condition:                               0.48          ⁢                      [Si]                          +                  [Cr]                                      30          ⁢                      [C]                          +                  18          ⁢                      [N]                          +                  0.01          ⁢                      [Mn]                                =                  from            ⁢              xe2x80x83            ⁢      0.8      ⁢              xe2x80x83            ⁢              to            ⁢              xe2x80x83            ⁢      1.3        ,
where [Si], [Cr], [C], [N], [Mn] is the content in the steel of silicon, chromium, carbon, nitrogen, and manganese, respectively, expressed in % by weight.
According to the invention, the proposed steel has a single-phase austenitic structure, the yield point 00.2 from 700 to 900 MPa, the breaking point "sgr"B from 1100 to 1250 MPa, obtained after water quenching at a temperature of from 1190 to 1230xc2x0 C. or obtained after water quenching at a temperature of from 1190 to 1230xc2x0 C. and subsequent tempering at a temperature of from 400 to 430xc2x0 C. for 3 to 3.5 hours with subsequent cooling in air.
Owing to the invention, the claimed nonmagnetic steel having a single-phase structure possesses a high mechanical strength, high corrosion- and wear-resistance, plasticity, and is inactive with respect to human tissues.
Th Further objects and advantages of the claimed invention will become clear from the following detailed description of the proposed austenitic steel and examples of its particular composition.
A nonmagnetic steel with the single-phase austenitic structure has been developed, that has a high mechanical strength, plasticity, corrosion- and wear-resistance, suitable for the manufacture of products to be used in medical engineering, for instance, prostheses, implants, medical tools, and the like.
The steel claimed in the present invention contains from 0.01 to 0.04% by weight of carbon, from 21.00 to 24.00% by weight of chromium, from 0.25 to 0.65% by weight of silicon, from 0.25 to 0.70% by weight of manganese, from 1.00 to 1.40% by weight of nitrogen, the balance being iron, the total content of ferrite-forming components in the steel, namely, of silicon and chromium, and the total content of austenite-forming components therein, namely, of carbon, nitrogen and manganese, obeying the following condition:                               0.48          ⁢                      [Si]                          +                  [Cr]                                      30          ⁢                      [C]                          +                  18          ⁢                      [N]                          +                  0.01          ⁢                      [Mn]                                =                  from            ⁢              xe2x80x83            ⁢      0.8      ⁢              xe2x80x83            ⁢              to            ⁢              xe2x80x83            ⁢      1.3        ,
where [Si], [Cr], [C], [N], [Mn] is the content in the steel of silicon, chromium, carbon, nitrogen, and manganese, respectively, expressed in % by weight.
The results of our investigations have shown that with the content of nitrogen in the steel less than 1.0% by weight, homogeneous y-solid solution (austenite) cannot be obtained in its structure, whereas with the content of nitrogen exceeding the claimed 1.4% by weight the conditions of melting and working the steel become complicated; the presence of nitrogen in the specified amount makes it possible to increase the yield point of the steel by as much as 2 to 3 times and to rule out introducing nickel and manganese, added heretofore to steel for these purposes, these additives inducing allergic reactions when in contact with human tissues. The content of chromium in the claimed amount of from 21.00 to 24.00% by weight increases the corrosion resistance of the steel, and under the indicated melting conditions the solubility of nitrogen can be increased eight-fold. It is difficult to attain the content of carbon in the steel less than the claimed 0.01% by weight without additional metallurgical operations, which make the steel appreciably more expensive; with the content of carbon exceeding the claimed 0.04% by weight the conditions, the conditions of formation of the homogeneous structure of nitrogen austenite are substantially complicated by the process of separation of large particles of chromium carbide of Cr23C6 type along the grain boundaries or of the formation of carbonitrides which lead to lowering the plasticity of steel and its resistance to intercrystallite corrosion. The content of chromium less than the claimed 21.00% by weight complicates the conditions of melting the steel with the claimed nitrogen content, which, as it was indicated, ranges from 1.00 to 1.40% by weight and insures the attainment, after tempering, of a homogeneous austenitic structure of steel containing no xcex4-ferrite or xcex1-martensite ferromagnetic phases; with the content of chromium exceeding the claimed 24% by weight, the 6-phase and nitrides appear in the steel structure, which deteriorate the mechanical properties of steel and are soluble only at temperatures that are technically difficult to achieve.
Our investigations have shown that for obtaining stable austenitic structure of the claimed steel the ratio of the sum of ferrite-forming components, namely, of silicon and chromium, to austenite-forming components, namely, carbon, nitrogen and manganese, is also of importance. So, it was found that when                               0.48          ⁢                      [Si]                          +                  [Cr]                                      30          ⁢                      [C]                          +                  18          ⁢                      [N]                          +                  0.01          ⁢                      [Mn]                                 less than     0.8    ,
a completely austenitic structure of steel cannot be obtained, while in the case of                               0.48          ⁢                      [Si]                          +                  [Cr]                                      30          ⁢                      [C]                          +                  18          ⁢                      [N]                          +                  0.01          ⁢                      [Mn]                                 greater than     1.3    ,
xcex4-ferrite appears in the steel structure.
Water quenching at a temperature of 1190-1230xc2x0 C. is sufficient for the homogenization of the xcex3-solid solutionxe2x80x94at a temperature above 1230xc2x0 C. grain growth and the appearance of xcex4-ferrite are observed; at a temperature lower than 1190xc2x0 C. complete dissolution of nitrides which deteriorate the viscosity and plasticity of steel cannot be attained. Tempering from the temperature of 430xc2x0 C. for 3-3.5 hours does not lead to the decomposition and nitrogen depletion of austenite. At a temperature not exceeding 400xc2x0 C. the strength of steel is not impaired. Keeping for 3-3.5 hours is sufficient for ensuring homogeneity of the steel structure.
The austenitic steel claimed in the present invention has high physico-mechanical characteristics: the yield point ("sgr"0.2) is 700-900 MPa, the breaking point ("sgr"B) is 1100-1250 MPa, a considerable abrasive resistance at an elevated plasticity: xcex4 is 28-51%, "psgr" is 20.5-39.0%. Said characteristics provide an increased service life and reliability of constructions and products from such steel, including implants subject to high loads, for instance, coxofemoral endoprostheses.
The claimed steel is advantageous over the prior art in that the content of carbon which contributes to the formation of thrombi is minimized, nickel which may induce allergic reactions and eczema is absent, and the steel is non-magnetic (because ferromagnetic material actively reacts with blood containing iron ions).
Therefore, the claimed austenitic steel can be used as a highly strong, wear- and corrosion-resistant non-magnetic material in the manufacture of products related to medical engineering, e.g., prostheses, implants, medical tools, and the like, providing for shortening the postoperative period of patients, ruling out the origination of inflammatory processes.
The claimed steel will also find successful application in instrument-making, power plant engineering, diesel building, cryogenic technology.
For a better understanding of the present invention, examples of its particular embodiment are given hereinbelow.